Preheat oven to 350 degrees.

In a bowl, whisk together eggs, milk, ground mustard, lemon zest, salt, and pepper until well combined.

Lightly grease a pie plate.

Cover the bottom of the pie plate with croutons.

Arrange tomato slices over the croutons.

Crumble gorgonzola cheese evenly over the tomatoes.

Pour the egg mixture over the tomato and cheese layers.

Bake in the preheated oven for 30-40 minutes, or until the eggs are fluffy and set and the top is golden brown.

Let cool slightly before serving.